In recent years, there has been an increase in prepared foods presented on pans and/or trays for sale or catering. Consequently, there is increased demand for inexpensive lightweight disposable covers or lids for use with such pans and/or trays to provide a covered container. Such covers normally are dome-shaped and are made of see-through or transparent plastic materials in order for the food to be observed. To minimize cost, it is desirable for the cover to be made of a minimum amount of plastic material, but still have sufficient stability to protect the food.
To provide improved stability, it is known to provide stiffening ribs in the surfaces of such covers as shown in U.S. Pat. Nos. 3,690,902 and 3,303,964. However, such prior stiffening ribs do not provide satisfactory stability as the thickness of plastic material is decreased to reduce the cost of the covers.